NO320814B1 - Sealing additive, fracturing fluid and method for fracturing a subterranean formation - Google Patents

Description

This invention relates to the extraction of hydrocarbon fluids from underground formations. More particularly, it relates to a new additive combination for controlling fluid loss, for use in fracturing fluids, a new fracturing fluid containing

holds such an additive combination, as well as a method for fracturing where the new fracturing fluid is used.

When extracting hydrocarbon materials from underground formations, is

it is common practice, particularly in the case of low permeability formations, to fracture the hydrocarbon-bearing formation while providing flow channels to facilitate the production of the hydrocarbons to the wellbore. In such fracturing operations, a fracturing fluid is hydraulically injected into a well that penetrates the underground formation, and is pressed against the formation by means of pressure. In this method, the formation is forced to crack or fracture, and a plugging agent is placed in the crack. The crack provides improved flow of the recoverable fluid, including oil, gas or water, into the well. Although many different fracturing fluids have been used, fracturing fluids generally comprise a thickened or gelled aqueous solution in which are suspended particles of "propagant" which are substantially insoluble in the fluids of the formation. Particles of the plugging agent carried by the fracturing fluid remain in the formed crack, the crack thus being braced so that it is open when the fracturing pressure is relieved and the well is put into production. Suitable plugging materials

materials include, but are not limited to, sand, walnut shells, sintered bauxite or similar materials. As those skilled in the art will appreciate, the "plugged" fracture provides a larger flow channel to the wellbore through which an increased amount of hydrocarbons can flow, thereby increasing a well's production rate.

A common problem with many hydraulic fracturing operations is the loss of fracturing fluid into the formation's porous matrix, especially in formations with high permeability, e.g. formations with a permeability of more than 2 md. Loss of fracturing fluid is unfortunate, not only because of cost considerations, but especially because it limits the fracture geometry that can be produced in formation-

is of high permeability. Generally, loss of fracturing fluid depends on the properties of the rock in the formation, the properties of the fracturing fluid, the shear rate of the fracture and the pressure difference between the injected fluid and the pore pressure in the rock matrix. With regard to this, the properties of the fracturing fluid are the properties that the fluid in the formation has, and which, among other things, are affected by the

temperature and shear history the fluid has been subjected to during its passage down the borehole and through the fracture.

Thorough analysis of the problem of loss of fracturing fluid in formations with high permeability shows that it is necessary to reduce "splash"

("asked"). As used herein, the term "splash" generally refers to the volume of fluid lost during fracturing due to early leakage of fracturing fluid before pores in the formation can be plugged and/or an outer fifter cake forms on the surface. Various additives have previously been used for the fluid, most of which have been selected or designed for the rapid formation of an outer filter cake with low permeability, under little or no shear stress (usually referred to as static conditions) to cover the pores and stop splash. This method is unsatisfactory, since high shear stresses eliminate or severely limit the formation of outer fifter cake.

Generally, the higher the permeability of a rock formation, the larger the spatter is likely to be. However, it has been demonstrated that during hydraulic fracturing, spatter occurs mainly at or near the protruding tip of the crack, where new rock surface is formed. The shear stresses that the fracturing fluid exerts on the surface of the rock formation are greater near the tip of the crack, due to the narrower crack opening at this location. As indicated, the high shear stresses prevent the formation of outer fifter cakes of polymer and/or fluid loss additives by eroding the surface of the cake in contact with the fracturing fluid. Consequently, for a fluid loss additive to be effective, it must be able to stop spatter under high shear rates.

US patent 4,997,581 describes the use according to the state of the art of various inorganic solids, natural starch types and combinations of finely divided inorganic solids with natural starch types. All of these materials are considered by these patentees to be deficient in controlling fracturing fluid loss in moderate to high permeability formations.

Although these patentees attempt to provide an effective additive using mixtures of natural starches and modified starches, their mixtures have limited application. When, for example, it concerns formations with high permeability and high temperatures, e.g. 149°C, natural and modified starch types may not effectively plug the pores in the crack walls. Finally, additives that have been proposed by other researchers in the field, although they provide some regulation of fluid loss, often entail exceptionally large costs.

Consequently, there has been a need for an inexpensive additive or fracturing fluid that provides regulation of loss of fracturing fluid, and a method for fracturing a subsurface formation, characterized by reduced fluid loss, under various conditions that include both high permeability and high temperature. The invention fulfills this need.

The invention therefore relates to an additive for regulating fluid loss (sealing additive), and which comprises a granular starch material and finely divided mica, the weight ratio between the starch material and mica being from approx. 11:1 to approx. 1:14. The mixture for regulating fluid loss according to the invention contains from to approx. 15 to about 50% by weight, based on the total weight of the additive, of a further finely divided inorganic solid, or a mixture of such solids. The additive components according to the invention can be added directly to a suitable fracturing fluid, or to facilitate the design, the additive components according to the invention can be suspended in a suitable diluent or carrier fluid, the combination of the additive for fluid loss control and carrier fluid being then combined with the fracturing fluid. The invention thus further comprises a mixture for fluid loss control which comprises a carrier liquid containing from approx. 2 to approx. 45% by weight of granular starch material and from 8 to 25% by weight of finely divided mica, based on the total weight of the mixture, the weight ratio between starch material and mica being from approx. 11:1 to approx. 1:14.

In another embodiment, the invention comprises a method for fracturing an underground formation that is penetrated by a borehole, which comprises injecting a fracturing fluid mixture containing a granular starch material and finely divided mica, the weight ratio between starch material and mica being from approx. 11:1 to approx. 1:14, in the borehole and in contact with the formation, at a rate and pressure sufficient to fracture the formation, and in an amount sufficient to provide fluid loss control. The fracturing fluid used preferably also contains a finely divided inorganic solid or solids, from approx. 0.48 to approx. 1.80 grams per litre.

Any suitable granular starch type or mixture of starch types can be used in the invention. Accordingly, as used in the following, the term "starch material" includes one or more natural starch types, one or more chemically modified starch types, and mixtures of one or more natural and/or chemically modified starch types. Natural starch types that can be used in the invention include, but are not limited to, starches from potatoes, wheat, tapioca, rice and corn, the preferred starch being potato starch. Pre-gelatinized starch types are most preferably used, especially pre-gelatinized potato starch. Pre-gelatinized starch types can be obtained commercially, or they can be prepared by pre-gelatinization treatment. In the case of pre-gelatinization, the selected starch granules are heated in water to a point where the starch granules swell irreversibly. On cooling, this swollen structure is retained. The use of pre-gelatinized starch types provides an important advantage when it comes to the combination according to the invention, since these materials are stable at higher temperatures in the formation, e.g. up to 149°C. Chemically modified starch types are those obtained from natural starch types by chemical reaction between a natural starch

type and a suitable organic reactant. Chemically modified starch types that can be used in the invention include, but are not limited to, carboxymethyl starch, hydroxyethyl starch, hydroxypropyl starch, acetate starch, sulfamate starch, phosphate starch, nitrogen-modified starch, starch cross-linked with aldehydes , epichlorohydrin, borates and phosphates, and starch grafted with acrylamide, acryl-

amide, acrylic acid, methacrylic acid, maleic anhydride or styrene. Among the modified

preferred starch types are hydroxypropyl and carboxymethyl starch types

ket. Although the granule size of the starch particles is not critical, as commercially available sizes are suitable, a preferred range of dry particle sizes will be from approx. 5 to approx. 150 p.m.

The particular type of mica used in the invention is a question

about what is desired. As used herein, the term "mica" generally refers to natural and synthetic silicate materials of varying chemical composi-

ning, characterized by the fact that they can be split into thin species or plates that are flexible and elastic. Suitable mica types include muscovite, phlogopite, biotftt, zinnwadite and pegmatite. As indicated, the fine particle mica is required. Median particle size

sen of the mica is preferably less than approx. 50 pm, most preferably below 32 pm.

If an additional inorganic solid or solids are used (i.e

in addition to, and distinct from, mica) together with the primary components, the median particle IstøreIsen of this will also, as indicated, be suitably small, common-

show in the same area as the mica patricles, and preferably the median

the particle size must be below approx. 50 µm. Preferred finely divided inorganic solids include solids of silicon dioxide, limestone (CaCO 3 ), rock salt, alumina, talc and kaolin.

The ratio between starch material and mica, on a weight basis, will be in the range from approx. 11:1 to approx. 1:14, preferably from approx. 5:1 to approx. 1:7. If it an-

if additional inorganic solid(s) are added, the inorganic solid(s) may replace part of the starch or mica in the total solids content of the mixtures. The finely divided inorganic solid(s) will preferably have a weight ratio between such solid(s) and mica of from approx. 1:1

to approx. 5:1, as the weight ratio between the finely divided solids and the starch material is thus from approx. 7:1 to approx. 7:3. Although one does not wish to be bound by any inventive step, it is believed that the somewhat deformable starch particles will partially fill pore throats ("pore throats") in the formation, the mica particles, which are small plates, filling the rest of the constrictions or cavities. The invention thus provides a bimodal pore filling mechanism that is characterized by a deformable particle with improved resistance to a high shear stress fracturing fluid, together with small particles that can help seal the pores. As far as this is concerned, the aforementioned relationships between glimmer-

more and starch crucial, since the voids in the crack surfaces cannot be completely sealed at ratios between mica and starch that are significantly below those designated, and at insufficient ratios between starch and mica, sealing of the larger pore narrowings will possibly not be achieved. The further finely divided inorganic solids, when used, will mainly be used in formations with very high permeability, e.g. greater than 100 md, where they are advantageous on

due to its rigidity.

Optionally, but preferably, the mixture of starch material and mica is mixed with a surfactant to aid in the dispersion of the dry starch-mica mixture in the fracturing fluid. Suitable surfactants include surfactants with less HLB (lipophilic) in the HLB range approx. 1-

11, with the HLB range 4-10 being preferred. Representative suitable surfactants include sorbitan monooleate, polyoxyethylene sorbitan monooleate, ethoxy-

clay butanol and ethoxylated nonylphenol, as well as various mixtures of these surfactants. The surfactants will typically be used at a level of

from approx. 0.1 to 10% by weight, and preferably approx. 0.5-5% by weight.

In practice, the additive components according to the invention are normally dispersed using the surfactant in a suitable diluent or carrier fluid. Suitable carrier fluids include low toxicity mineral oil, diesel fuel, kerosene and mixtures thereof. The carrier and additive components will preferably be combined in such a way that the starch will be present in an amount of from approx. 2 to approx. 45% by weight, the mica being present in an amount of from approx. 8 to approx. 25% by weight, where all percentages are based on the total weight of the carrier and components. If additional inorganic solid(s) are present, they will be present in an amount of from approx. 8 to approx. 25% by weight, all percentages again being based on the total weight of the carrier and components. The combination of additive material plus carrier is then easily mixed with, or dispersed in, a fracturing fluid.

The particular fracturing fluid used with the additive components according to the invention is largely based on freedom of choice and forms no part of the present invention. Fluids can, for example, comprise non-crosslinked solutions of cellulose or guar gum, or they can be crosslinked borate, titanium or zirconium fluids, the particular fluid being selected being determined by considerations such as treatment temperature and concentration of proppant to be is carried. As those skilled in the art will appreciate, however, the fracturing fluid and additive materials must be compatible in such a way that they do not react with each other or otherwise adversely affect the stated functions of each. The additive materials according to the invention are preferably used with water-based fracturing fluids, although this is not a requirement. Particularly preferred are the type of fracturing fluids described in US patent 5,259-455, and those described in US patent 4,686,052.

As indicated, the amount of additive components provided in the fracturing fluid will be the amount sufficient or effective to provide the desired control of fluid loss. This concentration of additive will vary depending on the particular formation's permeability and other properties. Typically, approx. 1.2-9.0 g/l of the additive components according to the invention in the fracturing fluid, with approx. 2.4-7.2 g/l of the additive components represents a preferred addition range. As indicated, the concentrations of each of the additive components in the fracturing fluid and the ratios between these are important if effective sealing of the pores is to be achieved. Typically, the fracturing fluid will

contain from approx. 0.24 to approx. 3.36 grams of starch material, and from approx. 0.3 to approx.

3.36 grams of mica per liters of fracturing fluid. If a further inorganic solid or solids is used, the concentration of such (such) solids) will be in the range from approx. 0.48 to approx. 1.8 grams per liter of fracturing fluid, preferably from approx. 0.6 to approx. 1.20 grams per litres.

After practicing the invention, as the fracture forms in the formation, the fluid loss control additive is deposited in the pores of the fracture walls forming a seal that regulates the rate of leakage and confines the fracturing fluid to the fracture. With the same fluid volume, a longer crack can therefore be obtained. Contrary to what would be expected, tests again show that the use of fracturing fluids with lower viscosity, which contain the additive components according to the invention, provides better regulation of fluid loss than when more viscous fluids are used.

In order to determine the properties of regulating fluid loss in the mixtures according to the invention, the following experiments were carried out. The experiments were carried out in dynamic fluid loss cells which were modifications of the unit described by L.P. Rood-hart in SPEJ, (October 1985), pp. 629-636. In the modified cells, measurements of dynamic fluid loss were carried out while the test fluid flowed in slot geometry, with an annular area in only one of the slot walls being porous. In each case the surface area (4.97 cm<2>) and length (2.54 cm) were the same for the core used. The width of the gap was the same as the diameter of the core. The variables for each run were thus temperature, pressure, core type and permeability, and shear rate.

In the experiments, aqueous fracturing fluids of the guar gum-containing type were made, which contained starch material and mica, or starch, mica and silicon dioxide flour, in the proportions indicated below. The starch, mica and silicon dioxide, if present, were first slurried with a small amount of No. 2 diesel, organophilic clay and surfactant for convenient dispersion in the fracturing fluid. Each fracturing fluid contained typical additives commonly found in such fluids, such as antifoam, bactericide, friction reducing agent and retarding agent. In the tables of results for each test, however, in order to demonstrate the significance of the additive components according to the invention, only comparisons have been made with runs of identical or analogous fracturing fluid that does not contain the additive components, under the same or substantially similar test conditions, as the only significant differences when it comes to the "controir runs" are the absence of the organophilic clay and surfactant, and a somewhat lower content of diesel no. 2.

In all cases, the shear rate was varied, as follows

The results of the experiments, with relevant variables, are as follows

IN

In these runs, a Barea sandstone core with a specific permeability of 1.90 was used, and the temperature was 65.5°C. In column A, the components of the control fracturing fluid mixture are listed, while the components according to the invention are listed in column B.

Fluid loss amounts (total) in milliliters, after the indicated time periods, were as follows:

At low specific permeability, the mixture according to the invention consequently shows improved regulation of fluid loss.

etc

In this set, the Barea sandstone "control" core had a specific permeability of 2.08, while the core used with the composition of the invention had a specific permeability of 2.03. The temperature used was 121°C, and the amount of guar gum was increased to 3.6 g/l. All other parameters were the same as for run I.

Amounts of fluid loss (total), in milliliters, were as follows

S

In this run, the Barea sandstone "controH" core had a specific permeability of 9.85, while the core used with the composition of the invention had a specific permeability of 10.11. All other parameters were the same as for run I. Amounts of fluid loss (total), in milliliters, were as follows

IV

The variables in this set correspond to the variables in run II, except that the Barea sandstone "controH" core had a specific permeability of 45.86, and the core used with the composition of the invention had a specific permeability of 49.28 . Amounts of fluid loss (total), in milliliters, were as follows:

Y

In these runs, the control core was a Barea sandstone core with a specific permeability of 200.42, the core used with the composition of the invention had a specific permeability of 199.60, and the temperature was 65.5°C. In column A is the "control" with the base fracturing fluid mixture shown, while column B defines the fluid of the invention.

Fluid loss amounts (total) in milliliters, after the indicated time periods, were as follows:

WE

In this run, the Barea sandstone "control" core had a specific permeability of 407.13, while the core used with the composition of the invention had a specific permeability of 404.93. All other parameters were the same as for Run V. Amounts of fluid loss (total), in milliliters, were as follows:

In summary, these experiments show good regulation ability in terms of fluid loss at high shear rates and over a wide spectrum of specific permeability and temperature.

Claims (8)

Additive for fluid loss regulation (sealing additive), characterized in that it comprises a granular starch material and fine-particle mica, the weight ratio between starch material and mica being from approx. 11:1 to approx. 1:14. 2. Additive according to claim 1, characterized in that it contains from approx. 15 to approx. SO weight%, based on the total weight of the additive, of a further finely divided inorganic solid or solids. in 3. Mixture for fluid loss regulation, characterized in that it comprises a carrier liquid containing from approx. 2 to approx. 45% by weight of granular starch material and from 8 to 25% by weight of finely divided mica, based on the total weight of the mixture, the weight ratio between starch material and mica being from approx. 11:1 tii approx. 1:14. 4. Mixture according to claim 3, characterized in that it contains from approx. 8 to approx. 25% by weight, based on the total weight of the mixture, of a finely divided inorganic solid or solids. 5. Fracturing fluid mixture according to claim 3, characterized in that it comprises a fracturing fluid which contains from in approx. 0.24 to approx. 3.36 grams of starch material and from approx. 0.30 to approx. 3.36 grams of mica per liter of fracturing fluid, as the weight ratio between starch material and mica is from approx. 11:1 to approx. 1:14. 6. Fracturing fluid mixture according to claim 5, characterized in that it contains from approx. 0.48 to approx. 1.20 grams of finely divided inorganic solid or solids per liters of fracturing fluid. 7. Method for fracturing an underground formation that is penetrated by a borehole, characterized in that it comprises injecting a fracturing fluid mixture comprising starch material and fine-particle mica, the weight ratio between starch material and mica being from approx. 11:1 to approx. 1:14, in the borehole and in contact with the formation, at a rate and pressure sufficient to fracture the formation, in an amount sufficient to provide fluid loss control. 8. Method according to claim 7, characterized in that the fracturing fluid mixture contains from approx. 0.48 to approx. 1.80 grams per litres, based on the mixture, of a further finely divided inorganic solid or solids.